Preheat the oven to 400°F. While the oven is warming, uses a large skillet over medium heat to sauté the onions and a couple pinches of salt in the oil/clarified butter. Cook, stirring regularly, until the onions are deeply golden and caramelized, 10 to 15 minutes. Remove from heat.

While the onions are cooking, cut any larger tomatoes in half. I typically leave the small cherry tomatoes whole. Add to the caramelized onions along with the sea salt and balsamic vinegar. Transfer to a 10 or 11-inch cast iron skillet or equivalent deep pie dish. If you get the sense that your tomatoes are quite juicy, and might release a lot of liquid, you can toss the mixture with a tablespoon or two of flour at this point. Sprinkle mixture with lemon zest.

Roll out your pie dough, and use it to cover the tomato mixture - tucking in the sides a bit. Brush the crust with the egg wash, cut a few decorative slits in the crust, and bake in the top third of the oven until the crust is deeply golden and the tomatoes are bubbling a bit at the sides, 25 to 30 minutes.